The nematodes are invertebrates belonging to phylum Nematoda. They have unsegmented thread-like body. Many of them are parasitic, living inside their host. Together with other parasitic worms such as flatworms, cestodes, and trematodes, the nematodes belong to a group of helminths. The nematodes are distinguished from other helminths by their thread-like unsegmented body, tough outer cuticle, and the presence of a tubular digestive system with openings at both ends.

Many of the species are disease-causing. The infection of the nematode Ascaris lumbricoides is characterised by an early pulmonary phase related to larval migration and a later, prolonged intestinal phase. Adult worms are 15-40 cm in length and maintain themselves in the lumen of the small intestine. Infection occurs after ingesting eggs contained in contaminated food or more commonly, by transmission to the mouth by the hands after contact with contaminated soil. Treatment is with mebendazole or pyrantel pamoate. Other parasitic nematodes include filarias, hookworms, pinworms, and whipworms.

Free-living species feed on algae, fungi, fecal matter, dead organisms, etc. An example of a free-living nematode is Caenorhabditis elegans that lives in the soil.